The real parts of third order optical nonlinearity of a series of azo dye (2 2 thiazolylazo 5 diethylaminophenol) and its copper and cobalt substituted complexes were measured by using femtosecond optical heterodyne optical Kerr gate technique at the wavelength of 647nm. All the values were found to be negative and great enhancement of nonlinearity was discovered in Cu and Co substituted complexes. By using a two level model, the enhancement is attributed to the two effects, i.e., the enhancement of one photon resonance of transition and the reduction of dipole difference between transition states. A negative γ eff as large as 10 -29 esu was found in Co complex.